1. Field of the Invention
The present invention generally relates to a polishing pad and a method of forming the same. More particularly, the present invention relates to a polishing pad having grooved window therein and a method of forming the same.
2. Description of Related Art
Endpoint detection techniques have been generally used for chemical-mechanical polishing (CMP) to ensure the polishing quality. Among those techniques, optical endpoint detection through a transparent detection window in a polishing pad provides a reliable result.
U.S. Pat. No. 5,893,796 discloses a method of forming a transparent window in a polishing pad by forming an aperture in a polishing layer first, and then a transparent window is fixed in the aperture with a layer of adhesive. Unfortunately, the slurry solution inevitably permeates through transparent window/polishing layer interface and leaks into the backside of the polishing pad, especially under the influence of the stress during polishing. Meanwhile, this would interfere with the optical signal and therefore the endpoint detection cannot be accurate.
U.S. Pat. No. 6,171,181 discloses a polishing pad with an integral window without above-mentioned slurry solution leakage problem. However, thereafter, during the process of forming grooves on the polishing pad with an integral window, grooves are also formed on the window portion with same groove depth as the grooves formed in the non-window portion. The non-smooth surface of the window portion, with grooves thereon, will lead to less light permeability and get lower reflected optical signal, which will degrade sensitivity of the endpoint detection. Besides, byproducts produced by reaction between polished materials and slurry and the slurry abrasive accumulated in the grooves on the window portion further makes endpoint detection difficult, even when the lifetime of the polishing pad is still within specification.
It is possible to form grooves by using mechanical techniques, such as cutting, only on the non-window portion of the polishing pad by controlling the distance between cutting tool and polishing pad, that is, the cutting tool lifts up only for the widow portion. However, the distance controlling stability for aligning to the window portion is challenging. Furthermore, there are some drawbacks of the polishing pad with no grooves formed on the window portion, firstly, sharp rising step of groove depth from grooved non-window portion to non-grooved widow portion will become a defect source when substrate moves relatively across window portion. Secondly, uneven friction and poor wettability of the window portion relative to non-window portion, due to absence of grooves on the entire window surface, makes the window itself become another defect source.
It would be desirable if a polishing pad having a transparent window therein that normally functions for endpoint detection can be utilized, while at the same time, the defects described above can be reduced or resolved.